The expanded executive committee of the Palestine Liberation Organization described President Reagan's plan for Middle East peace negotiations as "unacceptable," conference sources said today on the eve of a meeting of the PLO's parliament-in-exile.

But the sources said that moderates backing PLO Chairman Yasser Arafat at the committee's fourth consecutive late-night session could yet remove other draft language rejecting the U.S. plan outright, as the radical minority has demanded. The committee is drafting a series of resolutions to offer the Palestine National Council, and PLO leaders had hoped to reach full agreement on all major issues involved before the council meeting opens Monday.

Salah Khalaf, one of Arafat's top lieutenants who is better known as Abu Iyad, told reporters at the Club des Pins seaside conference site, that "we reject" the Reagan plan. But the drafting committee's policy was still unclear since other officials did not use such unequivocal language. Significantly, a spokesman for the radical Democratic Front for the Liberation of Palestine, when pressed to confirm Abu Iyad's wording, simply repeated that the Reagan plan was "not acceptable."

Khalil Wazir, better known as Abu Jihad and considered Arafat's second in command in the mainstream Fatah group, told reporters that the Reagan plan was "insufficient," but "we have to face it."

Such ambiguous, compromise wording would leave the door ajar, if only slightly, for future negotiations on the basis of the president's Sept. 1 plan, which called for a Palestinian entity on the Israeli-occupied West Bank and Gaza Strip in association with Jordan. Israel has rejected the plan repeatedly.

U.S. officials had hoped that the Palestinian council would simply make no mention of the Reagan plan in order to leave Arafat and King Hussein of Jordan as free as possible to work out a negotiating strategy.

Arafat has argued that the Reagan plan had "positive elements" but was incomplete. Palestinians were pleased by parts of the U.S. plan that insisted on Palestinian national rights and a freeze on new Israeli settlements, and by Reagan's refusal to countenance Israeli annexation of the territories occupied in the 1967 Arab-Israeli war.

But the Palestinian leaders objected to the plan's failure to mention the PLO's claim to be the sole legitimate representative of the Palestinian people and its objective of self-determination, and Reagan's rejection of the establishment of an independent Palestinian state on the occupied territories.

The drafting committee was reported in effect to have blocked any immediate negotiations between Jordan and Israel as envisaged by the Reagan plan. The conference sources insisted that any links with Jordan be in the form of a confederation once an independent Palestinian state has been established.

King Hussein favors a tighter, federal formula, and the United States opposes an independent Palestinian state and, unless the PLO recognizes Israel, any role for the group in negotiations. At most, the United States would accept tacit PLO backing for a delegation to negotiate with Israel.

Even pro-Arafat moderates are increasingly skeptical about American peace proposals. Their main argument in favor of moderation is based on a desire to show the world that Israel--not the PLO--has rejected peace talks out of hand, rather than on any faith in the president's formula or American forcefulness.

U.S. inability to persuade Israel to withdraw its troops from Lebanon has confirmed Palestinian pessimism and has convinced key PLO officials that Hussein would not dare agree to enter negotiatioins without their blessing.

Conference sources also said that the drafting committee, with one major proviso, had agreed to the Fez plan--which calls for an independent Palestinian state--as a minimum basis for negotiations. The plan was adopted by an Arab League summit in Fez, Morocco, in September.

The proviso, the sources said, seeks to nullify the heart of the Fez proposal, which indirectly recognizes Israel by guaranteeing the peace and security of all states in the region.

The exact language of the rider was not immediately available. The sources said Arafat might have his majority vote conspicuously in favor of the amended Fez plan.

Arafat can count on a large majority within Fatah, the mainstream group that he has fashioned as his power base since he first assumed control of the PLO in late 1968. Opposed to him are some Fatah radicals, Saiqa and other Syrian-backed groups and, to a lesser extent, George Habash's Popular Front for the Liberation of Palestine and Nayef Hawatmeh's Democratic Front for the Liberation of Palestine.

However, the dominant theme of this 16th meeting of the council is the absolute need for unity within the PLO after its defeat at Israeli hands in Beirut last summer.

Quite apart from that military setback, Arafat's free-wheeling stewardship has come under fire because of his exploratory negotiations with Jordan, his efforts to renew ties with Egypt--still suspect because of its separate peace with Israel--and his refusal to reject the Reagan plan outright.

About 350 delegates--40 more than attended the previous national council meeting in Damascus in April 1981--are taking part in this meeting, the first ever held outside the "frontline states," as the Arabs call countries bordering Israel.

The choice of Algiers was dictated by the PLO's desire to avoid holding the meeting in Damascus, Syria. Syrian President Hafez Assad and Arafat have not met since shortly after the PLO's evacuation from Beirut.